The present application relates generally to variable speed drives. The application relates more specifically to systems and methods for improved efficiency in variable speed drives having active converters.
A variable speed drive (VSD) for heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications typically includes a rectifier or converter, a DC link, and an inverter. The rectifier or converter converts the fixed line frequency, fixed line voltage AC power from an AC power source into DC power. The DC link filters the DC power from the converter and typically contains a large amount of electrical capacitance. Finally, the inverter is connected in parallel with the DC link and converts the DC power from the DC link into a variable frequency, variable voltage AC power.
Variable Speed Drives that incorporate active converter technology to provide power factor correction and reduced input current harmonics also generate a significantly higher level of common mode RMS and peak to peak voltage to the motor stator windings as compared to conventional Variable Speed Drives. This common mode voltage can be coupled to the rotor of the motor via various stray machine capacitances, causing motor and compressor bearing fluting, and these common mode voltages which result in currents flowing through the machine bearings may cause premature bearing failures in the motor and/or compressor.
Proper operation of the active converter control methodology, using the synchronous d-q reference frame requires knowledge of the instantaneous phase angle of the input line-to-line voltage. If the reference frame angle is incorrect or unknown, then the input power factor and the harmonic distortion of the input current to the Variable Speed Drive (VSD) with active converter cannot be controlled properly. If the VSD is required to ride-through an extended loss of the input line-to-line voltage and re-synchronize to the input mains when the power is restored, a means to retain the expected d-q reference frame angle during the loss of mains is needed. In addition, a means to quickly lock back onto the input mains line-to-line voltage and generate the actual phase angle of the line-to-line voltage is required.
Typical bypass means provided by drive manufacturers are active only when the VSD is incapable of running, in emergency situations. The bypass typically incorporates a minimum of two sets of three-phase contactors, one in series with the output of the inverter section and another between the incoming mains and the motor. In some cases a third set of three-phase contactors is implemented between the power mains and the input mains connection to the VSD. These bypass means are typically actuated via operator intervention via the drive keypad. Some suppliers may implement control means to provide automatic switchover to the bypass mode if the VFD fails. Some suppliers also provide “catch the spinning load” also called “windmill start” control means to catch and control an un-energized motor that is spinning, and bring it back up to full speed. Transfer from VSD operation to mains operation usually results in locked rotor torque being presented to the mechanical load and very high motor inrush current as the motor is started across-the-line.
In the past, VSD power assembly designs were bulky and heavy. They utilized aluminum electrolytic capacitors which have an inherent wear-out mechanism and are physically heavy and difficult to mount due to their cylindrical nature. The heatsinks were composed of either copper or aluminum material. Aluminum raises corrosion concerns when used in a closed loop uninhibited cooling system where copper components are also in intimate contact with the cooling fluid.
Typically, Low Voltage (less than 600 VAC) Voltage Source type Variable Speed Drives utilize air-cooled inductors, as the losses dissipated by the inductors are not high relative the losses in the remainder of the drive system. Also air-cooling is a less expensive option as compared to liquid cooling based on a de-ionized liquid style cooling loop. In addition, liquid cooling is often not available in the end use application.
Ground fault protection within a VSD can be implemented in various ways, e.g., an external ground fault sensor (a single “zero sequence” current transformer and detection circuitry) that opens a set of relay contacts or a molded case circuit breaker with a trip unit that incorporates a ground fault detection circuit. The level at which the ground fault current trip can be sensed is greater, and the accuracy of the sensed ground fault current is reduced, as a result of the three phase sensors. Another example of prior art ground fault protection employs motor current sensing means to shut down the inverter section of the VSD. This method does not provide ground fault protection for a ground fault occurring internally in the VSD.
Existing low voltage (i.e., less than 600 VAC) voltage source type VSDs utilize air-cooled inductors since the losses dissipated by the inductors are not high relative to the losses in the remainder of the drive system. In addition, air-cooling is often less expensive than liquid cooling. However, liquid cooling is often not available for end use applications. As active converter style VSDs are more widely used, the inductor losses may become more problematic as the size and cost of the required inductors may grow considerably.
What is needed is a system and/or method that satisfy one or more of these needs or provides other advantageous features. While the present invention is directed specifically to VSDs that incorporate an active converter type AC to DC converter topology, the invention is also effective for VSDs utilizing conventional AC to DC rectifier converters.
Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.